1. Field of the Invention
The present invention relates to a torsion module of a torque detection device of a steering system or of a steering power-assist system for a motor vehicle, the torque detection device including a spoked wheel that can be fastened to a steering wheel, the spoked wheel having a hub and a rim, the rim being arranged concentric to the hub and being joined to the hub by bending spokes.
2. Background Art
Electric steering power-assist systems are being used in motor vehicles to an increasing extent. The torque exerted on the steering wheel is required as a command variable, so that the desired steering power-assist can take place by using the ascertained torque. To detect the torque, a torsion module that permits a rotation angle offset between the steering wheel and the steering shaft when a torque is applied is used.
In the case of the device described in DE 27 34 182, an object is used as the torsion module, which is formed of two rings at a distance from each other in the axial direction. The two rings are connected to each other by metal strips placed at the same angular distance from each other. The top ring is connected in non-rotating fashion with the steering wheel and the lower ring is connected in non-rotating fashion with the steering shaft of the steering column. When a torque is applied to the steering wheel and thus to the upper ring, the metal strips experience a torsion and are subjected to bending. The extent of the bending of the metal strips provides information regarding the applied torque.
To ascertain this value, strip strain gauges that are attached to an evaluation unit are mounted on several of the metal strips. In the case of the object of this document, the metal strips that connect the two rings that are placed at an axial distance from each other are used as the actual torsion element. The disadvantage with the design of this torsion module is the relatively large overall height in the axial direction. It is exactly in this direction that very little space is available, especially in modern motor vehicles.
Additional disadvantages related to the object of DE 27 34 182 A1 are that movements of the steering wheel relative to the steering shaft in the axial direction also result in a deflection of the metal strips, with such a deflection being detected as a torque present at the steering wheel. Accordingly, a triggering of the steering power-assist system takes place without a torque actually being present at the steering wheel. As a result, this system is especially unsuitable for use within the context of a steering system in which the actual deflection of the wheels is produced only by means of, for example, an electrical system.
Torsion modules are also known in which two elements are placed concentric with each other as described in DE 37 37 696 A1, for example. In the case of the object of this torsion module, an interior hub is connected by means of spokes configured as bending rods to an external rim that is placed concentric with the hub. Connecting parts that transmit the given movement to one measurement transducer are each placed axially from the elements that are movable relative to each otherxe2x80x94i.e., the hub and the rimxe2x80x94so that a rotation offset between the rim and the hub can be registered according to the eddy current principle. The measuring device known from this document also requires installation space in the axial direction that is not always available.
In the case of the object of DE 37 37 696 A1, the torsion module, or more specifically, the evaluation unit coupled with the torsion module, is relatively insensitive to axial movements between the steering wheel and the steering shaft. While the bending spokes also bend with such a movement, such a movement does not lead to the generation of a steering signal due to the measuring sensor that is used.
Torsion modules have also been used for measuring the braking torque in a motor vehicle, as is described in DE 196 50 477 C1, for example. In the case of the torsion module described in this document, strip strain gauges are placed on the bending spokes in order to detect the rotation angle offset between the hub and the rim. In order to limit the maximum possible rotation angle offset between the hub and the rim, limit stop spokes that protrude from the hub and that engage in a recess in the rim are provided alternately with the bending spokes, which limits the maximum rotation angle offset. The wheel of a motor vehicle can be fastened to the rim.
For this purpose, the rim of the torsion module is set back from the mounting surface for the tires that is formed by the hub, so there is no danger that after a wheel is mounted on the limit stop spokes of the hub, the hub will lie against the bending spokes. A limitation of the axial movements between hub and rim is neither provided for nor necessary with the measuring device known from this document, because such movements between the hub and the brake disc mounted on the rim do not occur.
Starting from the state of the art discussed above, the present invention is thus based on the task of making available a torsion module that satisfies the requirements relative to the axial installation space needed with a torque detection device of a steering system or steering power-assist system, and which does not lead to misinterpretations in the event the steering wheel is contacted by forces acting in axial directions.
According to the present invention, this task is solved in that a pressure-sensitive or expansion-sensitive measuring sensor that generates electrical output signals is placed directly on a region of a bending spoke that is subject to bending in the event of a rotation angle offset between the rim and the hub, and in that placed alternately with the bending spokes are bending-resistant limit stop spokes, each of which has a free end that protrudes radially from the hub and engages in a limit stop arrangement in order to prevent a metrologically effective (i.e., measurable) relative movement between the rim and hub in the axial direction.
In contrast to the object of DE 37 37 696 A1, in the torsion module according to the present invention the measuring sensors are located directly on the bending spokes so that the required axial installation space is determined solely by the extension of the rim and the bending spokes in this direction. Foil-like strip strain gauges are preferred as measuring sensors because they are relatively easy to handle. The strip strain gauges are connected to an evaluation unit that does not, however, need to be part of the torsion module, but can instead be located in a steering wheel module or a steering column module, for example.
The torsion module of the present invention is especially advantageous with the concentric arrangement between the rim, configured, for example, as a ring body and thus as a rim that completely surrounds the hub, and the hub, because these elements are placed essentially in one plane. To avoid a misinterpretation of measured values from the measuring sensors due to a bending of the bending spokes in response to a relative axial movement between the steering wheel and the steering shaft, such a bending of the bending spokes in the axial direction is easily counteracted through placement of limit stops.
Such limit stops can be realized, for example, in that placed alternately with the bending spokes are limit stop spokes, which protrude radially from the hub. Each of the limit stop spokes engages with its free end in a limit stop arrangement. Each limit stop arrangement includes two limit stops placed at a distance from each other in the axial direction. The inside width of the spacer opening essentially corresponds to the material thickness of a limit stop spoke. Such limit stops can be simply realized in that a spacer ring that bears an inward-pointing projection in the region of each of the free ends of a limit stop spoke is placed, for example, on both the top and bottom of the rim. In place of one or both of the rings with such projections, such a limit stop can also be formed by an additional element, for example, the base plate of a steering wheel.
To provide overload protection, it is useful to limit the possible rotation angle offset between the hub and the ring by means of limit stop spokes. For example, to realize such a rotational angle limit stop arrangement, two bulges that protrude inward from the rim can be provided. The bulges have a distance from each other such that the free end of a limit stop spoke can engage therein, and specifically, with play that corresponds to the maximum rotation angle offset between the rim and the hub.
A torsion module with such a very flat design is especially well-suited for placement in a steering wheel module of a steering wheel. With the steering wheel module, the torsion module can form the lower element that is connected with the base plate of a steering wheel. The base plate of the steering wheel is then connected to the rim, while the hub of the torsion module is mounted in non-rotating fashion to the steering shaft of a steering column. The evaluation unit for reading the measurement results of the measuring sensors, the strip strain gauges for example, is also usefully placed in the steering wheel module, which can simultaneously be the carrier for an airbag device.